'QE  "- 

T  434-i 


UC-NRLF 


f    MoBERN 

PEtRoGRAPHY. 


VILLIA/AS. 


03 


GIFT   OF 
MICHAEL  REESE 


1 


PUBLISHER'S    PREFACE. 


TV  yTANY  contributions  to  the  theory  or  the  practice  of  teaching 
•*•  are  yearly  lost  to  the  profession,  because  they  are  embod- 
ied in  articles  which  are  too  long,  or  too  profound,  or  too  limited 
as  to  number  of  interested  readers,  for  popular  magazine  articles, 
and  yet  not  sufficient  in  volume  for  books.  We  propose  to  pub- 
lish from  time  to  time,  under  the  title  of  Monographs  on  Educa- 
tion, just  such  essays,  prepared  by  specialists,  choice  in  matter, 
practical  in  treatment,  and  of  unquestionable  value  to  teachers. 
Our  plan  is  to  furnish  the  monographs  in  paper  covers,  and  at  low 
prices.  We  shall  continue  the  series  as  long  as  teachers  buy 
freely  enough  to  allow  the  publishers  to  recover  merely  the  money 
invested. 

Of  this  series  we  are  now  ready  to  announce  the  four  following  :  — 

Modern  Petrography. 

By  GEORGE  HUNTINGTON  WILLIAMS,  of  the  Johns  Hopkins  Uni- 
versity.    Price  by  mail,  25  cents. 

The  Study  of  Latin  in  the  Preparatory  Course. 

By  EDWARD  P.  MORRIS,  M.A.,  Professor  of  Latin,  Williams  Col- 
lege, Mass.    Price  by  mail,  25  cents. 

Mathematical  Teaching  and  its  Modern  Methods. 

By  TRUMAN  HENRY  S AFFORD,  Ph.D.,  Field  Memorial  Professor 
of  Astronomy  in  Williams  College.  \_Ready  in  August. 

How  to  Teach  Reading  and  What  to  Read  in  the 

Schools.     By  G.  STANLEY  HALL,  Professor  of   Psychology  and 
Pedagogy,  Johns  Hopkins  University.  [Ready  in  September. 


MODERN    PETROGRAPHY 


AN   ACCOUNT   OF 


THE  APPLICATION  OF  THE  MICROSCOPE 
TO  THE  STUDY  OF  GEOLOGY 


BY 

GEORGE   HUNTINGTON  WILLIAMS 

\  i 

ASSOCIATE  PROFESSOR  IN  THE  JOHNS  HOPKINS  UNIVERSITY 


BOSTON 

D.  C.  HEATH   &   CO.,  PUBLISHERS 
1886 


COPYRIGHT,  JULY  10,  1886 
BY  D.  C.  HEATH  &  Co. 


ELECTROTYPED 
BY  C.  J.  PETERS  AND  SON,  BOSTON. 


MODERN  PETROGRAPHY. 

AN    ACCOUNT    OF    THE    APPLICATION    OF    THE    MICROSCOPE 
TO   THE  STUDY   OF   GEOLOGY. 


IT  cannot  be  denied  that  the  terms  Petrography  and  Lith- 
ology,  which  only  within  very  recent  years  have  come  to 
occupy  a  really  important  place  in  American  geological  liter- 
ature, still  convey  but  a  vague  meaning  to  most  teachers  of 
natural  science.  Many  men  who  have  devoted  themselves 
altogether  to  the  study  of  geology  know  little  of  the  origin, 
aims,  or  capabilities  of  the  youngest  branch  of  their  profes- 
sion. That  the  scientific  study  of  the  crystalline  rocks  has, 
during  the  past  twenty-five  years,  rapidly  developed  in  Ger- 
many is  a  fact  of  which  any  one  may  easily  convince  himself. 
The  perfection  of  its  methods  already  vies  with  that  in  many 
other  older  departments  of  investigation ;  while  the  impor- 
tance of  its  results  have  long  since  secured  for  it  a  well 
recognized  place  among  the  descriptive  sciences.  Nor  has 
its  value  as  an  educational  discipline  been  overlooked. 
Nearly  all  of  the  German  universities  have  to-day,  if  not  their 
special  professor  of  petrography,  —  as  may  be  found  at  Heid- 
elberg, Munich,  Leipzig,  Berlin,  and  Vienna, — at  least  their 
regular  courses  of  lectures  on  this  subject,  and  their  labora- 
tories amply  equipped  for  its  pursuit. 

It  is  only  quite  recently,  however,  that  the  importance  of 
what  the  Germans  have  accomplished  in  this  direction  has 


4  MODERN    PETROGRAPHY. 

commenced  to  be  appreciated  in  this  country.  When  the 
United  States  Geological  Survey  of  the  4oth  Parallel,  undei 
Mr.  Clarence  King,  found  it  necessary  to  have  a  systematic 
study  made  of  its  collections  of  crystalline  rocks,  there  was 
no  American  prepared  to  undertake  such  a  task,  and  the 
work  was  intrusted  to  Professor  Zirkel  of  the  University  of 
Leipzig.  The  appearance  of  the  results  of  his  labors  in  an 
admirably  illustrated  quarto  volume,  entitled  Microscopical 
Petrography  of  the  Rocks  of  the  Fortieth  Parallel,  published  as 
Vol.  VI.  of  the  reports  of  the  survey,  in  1876,  first  opened 
the  eyes  of  most  geologists  in  America  to  the  new  and  prom- 
ising field  of  research.  Since  that  time  the  interest  mani- 
fested in  this  line  of  study  in  America  has  been  steadily 
on  the  increase.  The  geological  surveys  of  this  country 
are  already  realizing  the  great  value  of  accurate  petro- 
graphical  studies;  and  if,  indeed,  we  but  compare  the 
quantity  and  quality  of  unexplored  material  in  America  with 
that  in  Europe,  we  must  conclude  that  it  is  here  that  the 
study  of  rocks  is  destined  to  reach  its  highest  development. 
Much  that  is  excellent  has  already  been  accomplished  on  this 
side  of  the  Atlantic;  but  the  workers  are  few,  and  hereto- 
fore there  has  been  observable  too  little  of  the  rigid  scientific 
accuracy  which  comes  only  after  years  of  patient  labor.  We 
are,  however,  heirs  of  the  past,  and  it  is  only  fair  that  we 
should  profit  by  all  the  accumulated  experience  of  our  prede- 
cessors. What  is  above  all  things  necessary  to  those  entering 
upon  a  line  of  research  so  difficult  and  new  is  a  careful 
training  in  what  has  already  been  discovered,  that  labor  may 
not  be  spent  in  vainly  working  out  results  which  have  already 
been  attained  by  others.  To  judge  from  the  American  stu- 
dents *  who,  during  the  past  six  years,  have  done  more  or  less 

*  During  the  writer's  residence  in  Heidelberg,  1880-1883,  nearly  one- 
half  of  all  the  students  in  the  petrographical  laboratory  were  Americans* 
and  the  proportion  now  is  even  greater. 


MODERN    PETROGRAPHY.  5 

work  in  the  petrographical  laboratories  of  Germany,  the 
importance  of  such  a  training  has  not  been  overlooked ;  and 
the  constantly  increasing  value  of  American  petrographical 
work  abundantly  justifies  the  expenditure  of  time  necessary 
to  secure  it. 

Until  recently,  it  has  not  been  possible  for  a  student  to 
secure  a  satisfactory  preparation  in  microscopical  petrogra- 
phy without  going  abroad  for  it.  The  English  language  does 
not  yet  contain  a  satisfactory  text-book*  on  this  subject, 
although,  as  we  shall  see,  the  first  idea  of  applying  the 
microscope  to  the  study  of  rocks  originated  in  England. 

Regular  instruction  in  petrography  has  for  some  years 
past  been  given  at  Harvard  and  Columbia  Colleges ;  more 
recently,  the  attempt  has  been  made  at  the  Johns  Hopkins 
University  to  organize  a  petrographical  laboratory,  where,  by 
lectures  and  practical  work,  graduate  students  of  geology 
may  secure  a  thorough  acquaintance  with  all  the  methods 
and  results  of  foreign  investigators.  The  encouragement  with 
which  this  experiment  has  already  met  seems  to  indicate  that 
it  fills  a  need.  Nor  are  signs  wanting  that  other  American 
universities  mean  to  follow  this  lead  by  introducing  instruc- 
tion in  petrography  among  their  courses. 

In  view,  therefore,  of  the  steadily  increasing  interest  in 
this  new  branch  of  geological  research,  it  has  been  thought 
that  a  brief  account  of  the  origin  and  history  of  microscopi- 
cal petrography,  as  well  as  of  some  other  methods  of  rock- 
investigation  to  which  its  cultivation  has  given  rise,  might 

*  Lawrence's  translation  of  Von  Cotta's  work,  Rocks  Classified  and 
Described,  London,  1866,  contains  no  allusion  to  the  microscope;  while 
the  small  text-book  by  Rutley,  The  Study  of  Rocks,  1879,  is  to°  inaccurate 
and  too  short  to  be  of  much  use.  Nor  can  more  be  said  in  favor  of  the 
recent  translation  of  Dr.  Hussak's  book,  The.  Determination  of  Rock- 
forming  Minerals.  The  German  edition  of  this  work  is  not  satisfactory 
in  its  arrangement  or  reliable  in  its  statements,  and  the  translation,  in- 
stead of  being  an  improvement,  is  rather  worse  than  the  original. 


6  MODERN    PETROGRAPHY. 

not  prove   unwelcome  to  teachers  in  many  departments  of 
natural  science.* 

The  reason  why  petrography  has  so  recently  sprung  into 
prominence  is  not  because  its  importance  was  not  early 
recognized,  but  rather  on  account  of  its  great  practical  diffi- 
culties, which  have  only  within  the  past  two  decades  been 
successfully  overcome.  The  fierce  contests  between  Nep. 
tunists  and  Vulcanists,  from  which  the  very  science  of  geol- 
ogy sprung,  themselves  hinged  largely  on  different  hypotheses 
regarding  the  nature  and  origin  of  crystalline  rocks.  The 
followers  of  each  school  strained  every  nerve  to  fortify  their 
position,  and  the  new  sciences  of  chemistry  and  mineralogy 
were  made  to  contribute  their  utmost  to  both  sides.  Much 
was  speedily  learned  about  the  composition  and  mineral  con- 
stituents of  the  coarse-grained  rocks,  but  any  satisfactory 
information  regarding  those  which  were  fine-grained,  and 
apparently  homogeneous,  eluded  the  search  of  even  the  most 
thorough  and  patient  investigators.  It  was,  however,  about 
exactly  this  class  of  rocks  that  the  discussion  had  been  most 
bitter,  and  we  can  but  regard  with  admiration  the  time  and 
study  which  the  ablest  geologists  devoted  to  them.  Still,  the 
results  attained  were  very  small.  In  1815,  Cordier  finally 

*  Those  desiring  more  detailed  information  on  this  subject  will  do 
well  to  consult:  — 

H.  Fischer \  Chronologischer  Ueberblick  iiber  die  allmahliche  Einfuhr- 
ung  der  Mikroskopie  in  das  Studium  der  Mineralogie,  Petrographie,  und 
Palaeontologie,  1868. 

F.  Fouque,  La  petrologie  en  Allemagne.  Revue  scientifique,  1875,  No.  34. 

F.  Fouque,  Les  applications  modernes  du  microscope  a  la  geologic. 
Revue  des  deux  mondes,  July  15, 1879. 

F.  Zirkel,  Die  Einfiihrung  des  Mikroskops  in  das  mineralogisch- 
geologische  Studium.  Leipzig:  1881. 

A.  Stelzner,  Die  Entwickelung  der  petrographischen  Untersuchungs- 
methodenin  den  letzten  fiinfzig  Jahren.  (Isis  Festschrift.)  Dresden:  1885. 

/./.  H.  Teal,  The  Scope  and  Method  of  Petrography.  Nature,  March 
12,  1885. 


MODERN    PETROGRAPHY.  7 

proved  that  basalt  was  an  aggregate  of  several  minerals, 
which  he  succeeded  in  partially  isolating  by  most  laboriously 
washing  them  with  water.  This  discovery  was  greeted  with 
delight,  but  the  method  was  at  the  same  time  recognized  as 
incapable  of  general  application.  Aside  from  the  immense 
amount  of  time  which  it  required,  it  was,  at  best,  exceedingly 
imperfect,  on  account  of  the  slight  differences  in  the  specific 
gravity  of  the  component  minerals.  As  a  matter  of  fact,  it 
was  never  used  again. 

The  rapid  development  of  analytical  chemistry  during  the 
first  half  of  this  century  threw  much  light  on  the  relations  of 
different  crystalline  rocks,  but  even  this  could  not  furnish 
what  geology  most  needed  —  a  means  of  determining,  with 
certainty,  their  mineralogical  components  and  structure.  The 
interpretation  of  analyses  was  necessarily  vague,  since  they 
could  be  calculated  to  satisfy  many  different  aggregates  of  sil- 
icate minerals. 

In  the  light  of  our  present  knowledge,  we  wonder,  not  at 
how  little,  but  at  how  much,  the  patience  and  acumen  of  men 
like  Von  Buch,  Brongniart,  Cordier,  and  Naumann  were  able 
to  discover  in  regard  to  fine-grained  rocks  with  the  exceed- 
ingly primitive  means  at  their  disposal.  Their  wide  experi- 
ence and  keen  judgment  enabled  them  to  make  many  shrewd 
guesses,  which  have  since  been  verified,  but  they  neverthe- 
less failed  in  their  main  object,  viz.,  the  discovery  of  some 
method  for  establishing  the  truth  of  what  they  only  surmised. 
Their  disappointment  we  may  still  find  expressed  in  such 
names  as  dolerite  (deceptive),  aphanite  (not  apparent),  etc. 

What  the  earlier  geologists,  especially  in  Germany  and 
France,  so  earnestly  desired,  it  was  reserved  for  the  students 
of  microscopical  petrography  to  accomplish.  Not  even  to- 
day should  crystalline  rocks  be  studied  altogether  with  the 
microscope,  but  still  the  revival  of  interest  in  rock-study  is 
so  entirely  due  to  the  introduction  of  this  instrument,  that 


8  MODERN    PETROGRAPHY. 

modern  petrography  may  properly  be  designated  as  micro- 
scopical petrography.  Other  methods  • —  chemical,  physical, 
and  geological  —  are  indispensable,  but  it  is  doubtful 
whether  these  would  have  reached  their  present  state  of 
development  if  it  had  not  been  for  the  new  impetus,  first 
given  to  this  department  of  inquiry  by  the  application  to  it 
of  the  microscope. 

In  his  admirable  historical  review  of  the  introduction  of 
the  microscope  into  the  study  of  mineralogy  and  geology, 
Professor  Zirkel  mentions  many  attempts,  some  of  them 
nearly  as  old  as  the  discovery  of  the  instrument  itself,  to 
apply  it  to  the  investigation  of  natural  inorganic  substances. 
Minerals  were  microscopically  examined  as  early  as  1663. 
At  the  beginning  of  the  present  century,  Fleurian  de  Belle- 
vue  and  Cordier  studied  rock-powder  in  the  same  way,  but 
without  marked  success.  Sir  David  Brewster  early  used 
polarized  light  in  his  investigations  of  the  inclusions  in 
natural  crystals ;  while  in  the  year  1830,  William  Nicol,  the 
inventor  of  the  invaluable  calcite  prism  which  bears  his 
name,  even  prepared  thin  sections  of  petrified  wood  for 
microscopic  study.  In  1834,  Talbot  contrived  a  microscope 
provided  with  two  nicol  prisms  for  the  easy  production  of 
polarized  light ;  but,  notwithstanding  the  discovery  of  all 
these  instruments  and  methods,  it  occurred  to  no  one  how 
useful  they  could  be  made  to  geology  by  their  application  to 
the  systematic  study  of  rocks.  The  microscopical  examina- 
tion of  mineral  powders  and  splinters  in  reflected  light  nat- 
urally yielded  no  satisfactory  results  ;  and  it  was  not  until 
1850  that  H.  Clifton  Sorby,  Esq.,  of  Sheffield,  Eng.,  ex- 
amined the  first  rock-section  in  transmitted  light.  But 
despite  this  beginning,  the  sporadic  attempts  in  this  direc- 
tion during  the  next  twelve  years,,  both  in  England  and  on 
the  continent,  profited  but  little.  The  real  possibilities  of 
microscopical  petrography  were  not  in  the  faintest  degree 


MODERN    PETROGRAPHY.  9 

realized.  The  microscope  was  regarded  merely  as  an  amus- 
ing toy ;  the  main  effort  was  to  discover  with  it  something 
novel  and  curious,  not  to  apply  it  scientifically  to  the  solu- 
tion of  broad  geological  problems.  Even  Sorby's  papers, 
which  continued  to  be  most  suggestive  in  this  line  of  work, 
had  reference  only  to  very  special  points  ;  and  it  may  be 
doubted  if  his  greatest  service  was  not  the  transplanting  of 
his  ideas  and  methods  to  Germany,  where  they  were  des- 
tined to  rapidly  take  root,  and  bear  a  fruitful  harvest.  Pro- 
fessor Fouque'*  has  so  well  described  the  circumstances 
under  which  this  came  about,  that  Professor  Zirkel  himself 
has  seen  fit  to  quote  his  words  ;  and  we  cannot  do  better 
than  follow  his  example  :  "  En  1862,  il  (Sorby)  avait  entrepris 
avec  sa  mere  un  voyage  d'agrement  sur  les  bords  du  Rhin. 
Arrive'  a  Bonn,  il  fit  connaissance  d'un  e'leve  du  corps  des 
mines  de  Prusse,  nomme  Zirkel,  par  lequel  il  fut  accompagne 
et  dirige  dans  quelques  excursions.  Us  visiterent  ensemble 
1'Eifel,  le  Siebengebirge,  et  les  environs  du  lac  de  Laach. 
Chaque  jour,  chemin  faisant,  une  conversation  inte'ressante 
et  anirne'e  s'engageait  entre  le  touriste  et  son  guide  sur 
la  nature  des  roches  volcaniques,  sur  les  mineraux  qui 
les  composent,  et  sur  les  merveilleux  details  de  struc- 
ture que  le  microscope  y  re've'le.  Sorby  exposait  avec 
clarte  et  chaleur  les  magnifiques  re'sultats  de  ses  eludes. 
Le  soir,  apres  1'excursion  de  la  journe'e,  1'entretien  se 
prolongeait  encore.  Enfin,  de  retour  a  Bonn,  le  maitre 
improvise  mit  sous  les  yeux  de  son  jeune  auditeur  quel- 
ques preparations  microscopiques  qu'il  avait  apporte'es,  et 
lui  fit  appre'cier  par  lui-meme  la  nettete'  et  Timportance 
des  faits  qui  avaient  e'te  1'objet  de  leurs  longues  causeries. 
Quelques  jours  plus  tard,  en  quittant  Zirkel,  il  laissait 
en  lui  un  disciple  enthousiaste,  qui,  desormais  se  consa- 
crant  entierement  aux  etudes  de  ge'ologie  micrographique, 
*  Revue  des  deux  mondes,  July  15,  1879,  P-  4°9» 


IO  MODERN    PETROGRAPHY. 

allait  bientot  dans  cette  voie  marcher  de  decouvertes  en 
decouvertes,  grouper  autour  de  lui  un  essaim  de  travail- 
leurs,  et  devenir  Tun  des  savans  les  plus  celebres  de 
I'Allemagne." 

Incited  by  the  enthusiasm  of  his  friend,  and  fully  realiz- 
ing the  great  importance  of  the  microscopical  study  of  rocks 
to  geology,  Zirkel  undertook,  during  the  winter  of  1862-63,  in 
the  laboratory  of  the  geological  Reichsanstalt  at  Vienna,  the 
first  systematic  study  of  rock-sections  as  an  end  in  itself. 
Heretofore,  all  such  investigations  had  been  accidental,  or,  at 
most,  accessory  to  some  other  result  which  was  aimed  at. 
The  difficulties  in  the  way  were,  however,  very  great ;  and  at 
first,  as  is  usually  the  case  in  an  entirely  new  departure  in 
scientific  research,  but  little  interest  in  or  sympathy  with  the 
work  was  manifested.  The  minerals  in  transmitted  light 
under  the  microscope  exhibited  an  altogether  different  char- 
acter from  that  which  they  ordinarily  presented  when 
macroscopically  examined ;  and  the  task  of  gradually  recog- 
nizing them  in  their  new  guise  was  a  slow  and  discouraging 
one.  Nevertheless,  energy  and  patience  overcame  the 
obstacles.  Mistakes  were  constantly  made,  but  as  con- 
stantly corrected,  until,  at  length,  an  amount  of  experience 
was  accumulated,  which  fixed  with  tolerable  accuracy  the 
microscopical  characteristics  of  the  commonest  rock-forming 
minerals.  During  the  nine  years  following  the  beginning  of 
his  investigations,  Zirkel  published  a  series  of  important 
articles  in  the  Sitzungsberichte  der  Wiener  Akademie,  in 
PoggendorfFs  Annalen,  in  the  Zeitschrift  der  deutschen 
geologischen  Gesellschaft,  and  in  the  Neues  Jahrbuch  fur 
Mineralogie,  Geologic,  and  Palaeontologie,  which  disclosed 
the  progress  made  in  the  microscopical  identification  of  min- 
erals, as  well  as  numerous  observations  regarding  the 
impurity  of  apparently  homogeneous  minerals,  whose  in- 
clusions and  structure  had  never  before  been  suspected. 


MODERN    PETROGRAPHY.  II 

Nor  was  it  long  before  another  important  result  of  micro- 
scopical rock-study  was  attained,  viz.,  the  discovery  that 
several  mineral  species,  like  leucite,  nepheline,  apatite, 
sphene,  tridimite,  etc.,  generally  considered  very  rare,  pos- 
sessed, as  microscopic  rock-constituents,  a  wide  distribution. 
This  was  made  especially  apparent  in  Zirkel's  Basalt- 
gesteine,  published  in  1870,  in  which  he  found  it  necessary 
to  add  to  the  ordinary  feldspar-basalts  two  other  classes, 
characterized  by  their  containing  leucite  or  nepheline  as 
an  essential  constituent. 

But  it  must  not  be  imagined  that  during  all  these  years 
Zirkel  was  alone  in  his  microscopical  studies  of  rocks.  In 
1864  appeared  Laspeyres'  investigations  of  the  porphyries 
near  Halle ;  and  soon  after,  many  other  similar  papers  on 
volcanic  rocks,  by  Vom  Rath,  Kosmann,  Weiss,  Dressel, 
and  others.  Fischer's  Critical  Micromineralogical  Studies 
(1869-1873)  gave  the  results  of  a  large  number  of  observa- 
tions, which  showed  how  few  of  the  so-called  mineral  species 
possessed  really  homogeneous  crystals.  This  accounted  in  a 
satisfactory  manner  for  the  frequent  discrepancies  in  analy- 
ses of  the  same  mineral  from  different  localities,  or  by  differ- 
ent investigators.  But  perhaps  the  man  whose  work  during 
this  first  period  in  the  development  of  microscopical  petrog- 
raphy (1862-1873)  gave  the  most  promise  was  Hermann 
Vogelsang.  He  it  was  who  seemed  earliest  and  best  to 
realize  what  important  services  the  microscope  was  capa- 
ble of  rendering  to  geology.  In  1867,  his  Philosophy 
of  Geology  was  published,  in  which  the  third  division, 
entitled  "  Modern  Geology,"  was  devoted  to  microscopical 
observations.  This  is  even  at  the  present  time  a  most  valu- 
able and  useful  book.  It  is  written  in  a  charming  style,  is 
full  of  most  suggestive  ideas,  the  true  force  of  many  of 
which  is  only  now  commencing  to  be  thoroughly  appreciated, 
while  its  colored  plates,  whose  accuracy  and  beauty  have 


12  MODERN    PETROGRAPHY. 

never  yet  been  excelled,  make  it  still  one  of  the  most 
important  volumes  in  a  petrographical  library.  Nor  was 
Vogelsang  content  alone  to  observe.  He  was  a  born  experi- 
menter, and  was  constantly  striving  to  artificially  reproduce 
the  results  of  nature  under  circumstances  similar  to  hers, 
where  the  processes  could  be  studied.  In  this  he  was 
very  successful.  In  1868,  he  and  Geissler  proved  for  the 
first  time  the  presence  of  liquid  carbon-dioxide  in  quartz. 
Sir  David  Brewster  and  Sorby  had  long  before  suspected  it; 
but  Vogelsang  was  able  to  extract  the  liquid,  and  examine 
its  spectrum.  Later,  he  devoted  his  attention  to  the  study 
of  the  devitrification  products  formed  in  glassy  rocks,  compar- 
ing them  with  similar  forms  in  artificial  slags,  and  finally 
reproducing  their  various  characteristic  shapes,  by  allowing 
sulphur  to  crystallize  under  the  microscope,  where  the  forma- 
tion and  growth  of  the  crystals  were  retarded  by  Canada 
balsam.  The  results  of  these  most  interesting  studies  and 
experiments  were,  alas !  destined  to  be  given  to  the  world 
only  after  their  talented  author  had  quitted  it,  a  fact  which 
cannot  be  too  deeply  deplored,  as  it  deprived  the  young 
science  of  one  of  its  most  earnest  and  successful  cultivators. 
Vogelsang's  last  work,  edited  by  Professor  Zirkel,  and  pub- 
lished at  Bonn,  in  1875,  under  the  title  Die  Krystalliten, 
will  always  remain  a  monument  to  his  carefulness  and 
originality. 

In  the  year  1873  microscopical  petrography  entered  upon 
a  new  period  in  its  development.  The  new  science,  which 
at  first  was  regarded  as  something  curious  and  novel  rather 
than  as  anything  really  useful,  gradually  commenced  to  make 
itself  felt.  Older  geologists  began  to  realize  the  important 
role  it  was  destined  to  play,  while  the  younger  workers  were 
attracted  to  it  as  the  newest  and  least  occupied  field  of  dis- 
covery. Still,  any  thorough  knowledge  of  the  young  depart- 
ment was  confined  to  the  few  pioneers  who  had  themselves 


MODERN    PETROGRAPHY.  13 

shaped  its  growth.  The  need  of  a  reliable  text-book,  which 
should  contain  in  systematic  form  all  the  information  which 
had  been  gathered  regarding  the  identification  of  rock-form- 
ing minerals  under  the  microscope,  was  constantly  becoming 
more  pressing,  and  the  year  1873  witnessed  the  production 
of  two.  One  of  these,  by  Professor  Zirkel,  entitled  Die 
mikroskopische  Beschaffenheit  der  Mineralien  und  Gesteine^ 
brought  together  in  available  shape  all  the  results  thus  far 
attained,  which  were  widely  scattered  through  numerous 
periodicals.  The  other,  by  Prof.  Heinrich  Rosenbusch, 
called  Die  mikroskopische  Physiographie  der  petrographisch 
wichtigen  Mineralien,  gave  new  and  more  scientific  methods 
for  the  recognition  and  diagnosis  of  minerals  in  rock-sections 
than  had  ever  before  been  employed.  Rosenbusch,  who,  with 
his  inaugural  dissertation  on  the  nephelinite  of  the  Katzen- 
buckel,  had  only  a  short  time  previously  appeared  as  a 
worker  in  the  ranks  of  petrography,  was  destined  very  soon 
to  become  recognized  as  its  leader.  The  investigation  of 
the  optical  properties  of  crystals  had  long  occupied  the  atten- 
tion of  eminent  mineralogists.  Des  Cloizeaux  had  studied 
their  behavior  in  converged  polarized  light ;  Von  Kobell,  by 
his  ingenious  invention  of  the  stauroscope,  had  discovered 
a  means  of  readily  determining  the  position  of  the  axes  of 
elasticity  relative  to  the  crystallographic  axes ;  Haidinger 
had  especially  examined  the  phenomenon  of  pleochroism, 
while  Tschermak  had  published  comparative  optical  studies 
of  very  important  groups  of  rock-forming  minerals,  like  the 
feldspars  and  amphibole-pyroxene  family.  It  is  Rosenbusch's 
great  service  to  petrography  to  have  been  the  first  to  show 
how  these  various  optical  methods,  so  carefully  elaborated 
for  individual  crystals  when  cut  in  known  directions,  could  be 
applied  to  their  identification  when  occurring  in  confused 
aggregates  and  intersected  in  every  possible  direction.  The 
microscope  which  he  described  as  especially  fitted  for  petro- 


14  MODERN    PETROGRAPHY. 

graphical  work,*  was  provided  with  a  revolving  stage,  a  con- 
venient polarizing  apparatus,  and  lenses  for  securing  either 
converged  or  parallel  light  at  will.  It  still  remains  essen- 
tially unchanged  as  the  most  convenient  model.  Rosen- 
busch's  method  was  accurate,  and  in  nearly  every  case 
sufficient.  It  has  been  amplified  and  improved  in  many 
respects,  but  it  remains  to-day  the  basis  upon  which  all  the 
truly  scientific  claims  of  microscopical  petrography  must  rest. 
Before  1873  all  mineralogical  determinations  in  fine-grained 
rocks  had  been  hazardous  and  empirical;  it  was  only  the 
logical  development  and  application  of  the  established 
principles  of  optical  mineralogy  to  the  study  of  rock- 
sections  which  could  place  this  on  a  firm  and  established 
footing. 

After  the  publication  of  the  two  above-named  text-books 
in  1873,  the  position  of  petrography  as  a  department  of 
science,  and  even  as  an  educational  discipline,  was  not  only 
abundantly  assured,  but  its  growth  was  exceedingly  rapid. 
Other  text-books,  like  those  of  Von  Lasaulx,  Lang,  and 
Fouque'  and  Michel-Le'vy,  followed  each  other  in  quick  suc- 
cession. As  Vogelsang's  keen  insight  had  discerned  as 
early  as  1867,!  the  microscope  had  a  twofold  mission  to 
accomplish  in  the  study  of  rocks  during  this  period  •  first,  to 
secure  the  means  of  a  ready  and  certain  identification  of  the 
mineral  constituents ;  second,  to  investigate  the  structural  rela- 
tions of  these  constituents  to  each  other.  Both  of  these  would 
furnish  the  data  necessary  for  a  satisfactory  classification  of 
the  fine-grained  rocks,  before  in  such  hopeless  confusion. 
The  first  of  these  ends  had  been  in  a  measure  reached 
through  the  labors  of  Zirkel,  Tschermak,  and  especially 
Rosenbusch ;  the  second  was  rapidly  furthered  by  the 
numerous  papers  which  appeared  relating  to  comparative 

*  Neues  Jahrbuch  fiir  Mineralogie,  etc.,  1876,  p.  504. 
t  Philosophic  der  Geologic,  p.  131. 


MODERN    PETROGRAPHY.  15 

studies  of  rock-types.  Thus  the  observations  of  Hagge  on 
gabbro,  of  Harrmann  on  melaphyre,  of  Kalkowsky  on 
felsite-porphyry,  and  of  Dathe  on  diabase,  rendered  great 
service  in  fixing  the  precise  meaning  of  these  names  to 
certain  definite  mineral  aggregates  and  structure-forms. 
They  had  all  been  used  long  before,  but  with  such  different 
meanings  by  different  authors  as  to  possess  only  the  vaguest 
significance. 

So  general  did  the  interest  in  petrography  become  among 
the  German  geologists,  and  so  rapidly  did  microscopical 
observations  accumulate,  that,  in  the  year  1877,  Rosenbusch, 
by  uniting  what  others  had  accomplished  to  his  own  untiring 
studies,  was  able  to  add  a  second  volume  to  his  great  work, 
under  the  title,  Die  mikroskopische  Physiographic  der  massigen 
Gesteine.  This  book  contains  a  wealth  of  information 
respecting  the  mineral  composition,  the  structure  and  dis- 
tribution of  all  the  igneous  rocks,  both  plutonic  and 
volcanic.  It  proposed  a  system  of  classification  based  upon 
differences  of  geological  age  —  pre-tertiary  and  tertiary ; 
upon  the  degree  of  the  crystalline  structure — glassy,  por- 
phyritic,  and  granular;  and,  lastly,  upon  the  nature  of  the 
mineral  components.  This  classification  aimed  to  be  an 
improvement  on,  rather  than  a  substitute  for  the  systems 
already  in  use.  It  adopted  all  that  they  possessed  which 
was  in  accord  with  what  the  microscope  disclosed,  while  it 
corrected  and  supplemented  old  names,  which  had  long  been 
loosely  used,  by  assigning  sharp  meanings  to  them.  It 
introduced  new  names  only  where  it  was  altogether  necessary, 
and  in  general  it  did  as  little  violence  as  possible  to  old 
ideas.  These  facts,  coupled  with  the  evident  simplicity  of 
the  new  arrangement,  soon  secured  for  it  a  general  accep- 
tance among  petrographers,  who  had  long  felt  the  great 
necessity  of  some  uniformity  in  nomenclature.  Although 
not  in  all  respects  exactly  what  the  present  condition  of 


l6  MODERN    PETROGRAPHY. 

petrography  requires,  Rosenbusch's  classification  of  the 
massive  rocks  is  used  almost  exclusively  to-day  the  world 
over  as  the  most  available,  while  his  book  still  remains  the 
great  storehouse  of  information  regarding  their  microscopical 
characters.* 

It  would,  however,  be  anything  but  fair  to  the  modern 
methods  of  rock-study  to  imply  that  they  consist  altogether 
of  the  investigation  of  thin-sections  with  the  microscope. 
Great  as  has  been  the  influence  of  this  instrument  in  giving 
an  impetus  to  this  line  of  work  in  geology,  and  invaluable  as 
has  been  the  assistance  which  it  has  afforded,  petrography  is 
by  no  means  exclusively  confined  to  its  application.  Total 
and  partial  chemical  analyses  of  rocks  have  long  been  made 
and  discussed,  although  it  must  be  confessed  that  before  the 
microscope  was  used,  this  was  done  to  little  purpose.  Now, 
however,  rock-analyses  have  acquired  a  new  significance,  and 
are  indispensable  as  a  supplement  to  microscopical  study. 
When  the  minerals  present  are  definitely  known,  the  analysis 
may  ba  calculated  so  as  to  give  their  quantitative  proportions* 
In  some  cases  we  may  be  dependent  upon  the  analysis  for 
even  the  exact  determination  of  the  nature  of  a  rock,  as  in 
the  case  of  volcanic  glasses  or  certain  rhyolites,  where  the 
silica  is  in  an  amorphous  state. 

Furthermore,  the  microscope  has  rendered  possible  the 
development  of  certain  very  delicate  and  beautiful  micro- 
chemical  re-actions,  by  which  the  nature  of  an  otherwise 
doubtful  mineral  may  be  established.  Especially  is  this  the 

*  The  second  edition  of  this  great  work  of  Rosenbusch,  the  appear- 
ance of  which  is  announced  for  the  coming  summer,  may  be  relied  upon 
to  contain  an  exhaustive  account  of  all  that  is  now  known  regarding  the 
origin,  relationships,  and  classification  of  the  massive  crystalline  rocks,  in 
the  same  way  that  the  second  edition  of  his  first  volume,  published  last 
October  (1885),  COM  tains  a  complete  resume  of  all  that  has  heretofore 
been  discovered  regarding  the  identification  of  rock-forming  minerals 
under  the  microscope. 


MODERN    PETROGRAPHY.  I/ 

case  with  members  of  certain  isomorphous  groups  like  the 
feldspars  or  the  pyroxenes,  whose  microscopical  characters  are 
often  nearly  identical.  Many  of  these  microchemical  tests 
are  very  delicate  and  accurate.  Boricky  showed  how  the 
characteristic  crystals  of  the  salts  formed  by  a  drop  of 
hydrofluorsilicic  acid  with  the  alkaline  bases  of  a  silicate 
could  be  used  not  only  for  detecting  the  presence  of  these 
bases,  but  for  approximately  determining  their  relative  pro- 
portions.* Behrens  proposed  to  recognize  the  presence  of 
alumina  in  minute  mineral  fragments  by  transforming  its 
bases  into  sulphates,  and  then  bringing  them  in  contact  with 
caesium-chloride,  with  which  aluminium-sulphate  forms  the 
insoluble  caesium-alum. f  Haushofer  has  recently  published 
in  convenient  form  all  that  has  thus  far  been  discovered  in 
regard  to  microchemical  re-actions. t  Other  good  quali- 
tative re-actions  are  obtained,  according  to  the  method  of 
Szabo,  in  the  flame  of  a  Bunsen  burner  or  with  the  spectro- 
scope. 

It  will  be  readily  seen  that  the  accuracy  of  these  micro- 
chemical  re-actions  is  to  a  large  extent  dependent  upon  the 
possibility  of  completely  separating  the  constituent  minerals 
of  a  rock  from  each  other,  and  thus  obtaining  pure  material 
with  which  to  work.  In  the  case  of  the  finest-grained  rocks, 
at  least,  this  would  seem  to  present  very  considerable  diffi- 
culties. It  is  something  which  geologists  have  long  sought 
to  accomplish,  but  it  is  only  recently  that  such  efforts  have 
been  attended  with  success.  Now,  however,  the  methods 

*  Elemente  einer  neuen  chemisch-mikroskopischen  Mineral-  und  Ges- 
teinsanalyse  von  E.  Boricky.  Archiv  der  naturwissenschaftlichen 
Landesdurchforschung  von  Bohmen,  III.,  5.  Prag:  1877. 

t  Mikrochemische  Methoden  zur  Mineral-Analyse  von  H.  Behrens; 
Verslagen  der  kon.  Akad.  van  Wetenschappen.  1881  (2)  XVII., 
27-72. 

J  Mikroskopische  Reactionen  von  K.  Haushofer.  Braunschweig :  8° . 
1885,  pp.  162. 


1 8  MODERN    PETROGRAPHY. 

for  the  mechanical  separation  of  the  different  mineral  con- 
stituents of  even  the  compactest  rocks  are  hardly  inferior 
in  their  accuracy  and  completeness  to  any  of  the  others  em- 
ployed in  modern  petrography.  The  attempts  of  Cordier  in 
1815  to  separate  the  constituents  of  basalt  by  their  specific 
gravities  in  water  have  been  already  referred  to.  Although 
partially  successful,  they  were  so  laborious,  and,  on  the  whole, 
so  unsatisfactory,  that  the  method  never  came  into  general 
use.  No  real  advance,  indeed,  was  made  toward  the  attain- 
ment of  this  much-desired  result  until,  in  1878,  Thoulet  sug- 
gested the  application  of  a  concentrated  solution  of  the 
iodides  of  mercury  and  potassium  (whose  specific  gravity  was 
about  3),  for  the  separation  of  the  mineral  constituents  of  a 
finely  pulverized  rock.*  Sonstadtf  and  Church  J  had  pre- 
viously described  the  use  of  this  solution  as  especially  valuable 
for  readily  determining  the  specific  weight  of  minerals,  so  that 
here  again  we  can  trace  the  first,  idea  of  one  of  the  most 
important  petrographical  methods  to  England.  The  physical 
properties  of  this  solution  were  very  fully  investigated  by 
Goldschmidt,  in  i88o,§  and  its  maximum  density  found  to  be 
as  high  as  3.19.  Still,  however,  the  separation  of  rock-con- 
stituents was  only  partially  attained,  inasmuch  as  many  of  the 
most  important  minerals,  like  hornblende,  pyroxene,  olivine, 
garnet,  etc.,  were  heavier  than  the  solution  in  its  most  con- 
centrated form.  The  discovery,  therefore,  by  Klein,  ||  in  1881, 
of  another  salt  which,  in  concentrated  solution,  possessed  a 
specific  gravity  of  3.28  at  ordinary,  and  of  3.6  at  higher  tem- 
peratures, was  gladly  received  as  a  great  improvement  on  the 
one  in  use.  Klein's  solution,  which  is  a  borotungstate  of  cad- 

*  Comptes  rendus.     Feb.  18,  1878. 
t  Chemical  News,  1874.     XXIX.,  p.  127. 
J  Mineralogical  Magazine,  1877.     I.,  p.  237. 

§  Neues  Jahrbucjj  fur  Mineralogie,  etc.,  I  Beilage  Band  1881,  p.  174. 
||  Comptes   rendus,   1881.    XCIIL,  p.  318;   and  Bull.  Soc.  Min.  de 
France,  IV.,  p.  149. 


//  i  OF  rut        *r  N\ 

u  UNIVERSITY)) 

MODERN    PETROGRAPHY.  IQ 

mium,*  rendered  possible  for  the  first  time  the  almost  com- 
plete isolation  of  nearly  all  the  minerals  which  enter  into  the 
composition  of  rocks.  Still  more  recently,  Rohrbach  f  has 
proposed  the  use  of  a  solution  of  the  iodides  of  mercury 
and  barium,  which,  though  easily  decomposed,  possesses  the 
great  advantage  of  having  a  specific  gravity  of  3.588  at 
ordinary  temperatures. 

The  method  of  procedure  with  all  of  these  heavy  solutions 
is  the  same.  The  rock-powder  is  rendered  uniform  by  sift- 
ing, a  grain  of  from  a  one-hundredth  to  a  one-hundred  and 
fiftieth  of  an  inch  being  found, %  in  general,  most  serviceable 
for  fine-grained  rocks.  The  solution  in  its  most  concentrated 
state  is  brought  into  an  apparatus  like  the  separating-funnel 
used  by  chemists, §  and  the  powder  well  shaken  up  in  it. 
After  standing  a  short  time,  all  those  grains  which  are  lighter 
than  the  liquid  will  rise  to  the  top,  while  those  which  are 
heavier  will  sink,  and  may  be  drawn  off  with  the  loss  of  but 
little  liquid.  If,  now,  the  liquid  be  gradually  diluted  by  the 
addition  of  water,  points  will  be  successively  reached  where 
the  different  minerals  will  fall  in  the  order  of  their  specific 
gravities.  As  each  is  precipitated,  it  may  be  drawn  off  as  in 
the  first  instance,  and  in  this  manner  all  the  constituents  of 
the  rock  which  are  lighter  than  the  concentrated  solu- 
tion may  be  obtained  in  quite  a  pure  state.  The  specific 
gravity  of  the  liquid  at  each  point  where  a  mineral  falls  may 
readily  be  determined  by  a  Mohr's  balance,  as  suggested  by 

*  The  formula  for  this  is  :  —  9WO3,  B2O3,  2CdO,  2H2O  +  16  aq. 

t  Neues  Jahrbuch  fur  Mineral ogie,  etc.,  1883,  II.,  p.  186;  and  Poggen- 
dorff' s  Annalen  (neue  Folge),  XX.,  p.  167  (July,  1883). 

t  The  writer  has  found  bolting-cloth  very  serviceable  in  the  prepara- 
tion of  such  sieves.  It  is  very  tough,  regular  in  mesh,  and  may  be 
obtained  of  almost  any  degree  of  fineness. 

§  An  apparatus  admirably  adapted  for  this  purpose  was  contrived  by 
T.  Harada  and  described  by  Oebbeke,  Neues  Jahrbuch  fiir  Mineralogie, 
etc.,  I  Beilage  Band,  p.  457. 


2O  MODERN    PETROGRAPHY. 

Cohen ;  *  or  by  the  introduction  into  the  solution  of  so-called 
indicators  —  bits  of  mineral  whose  specific  gravity  has  been 
accurately  determined.  The  solution  used  for  each  such 
separation,  as  it  is  somewhat  expensive,  is  carefully  pre- 
served, and,  together  with  the  water  in  which  the  powders 
are  washed,  is  filtered  and  restored  to  its  original  state  by 
simply  evaporating  it  on  a  water-bath. 

Several  other  methods  of  separating  the  component  min- 
erals of  rocks  have  also  recently  been  elaborated,  and 
under  certain  circumstances,  yield  excellent  results.  Profes- 
sor Fouque  made  use  of  the  different  degrees  of  resistance 
which  the  various  rock-forming  silicates  offer  to  the  action  of 
hydrofluoric  acid  for  obtaining,  in  a  pure  state,t  the  hyper- 
sthene  from  the  lavas  of  Santorin.  The  other  minerals  present 
were  dissolved  before  the  hypersthene  was  appreciably 
affected.  Rutile  disseminated  through  certain  schists  in 
crystals  of  most  microscopic  dimensions  has  also  been  suc- 
cessfully isolated  in  the  same  manner.}: 

Magnetic  minerals  can  readily  be  extracted  with  an  ordi- 
nary magnet;  while,  according  to  Doelter,§  an  electro-magnet 
may  be  employed  to  separate  those  having  much  iron  in  their 
composition  from  those  which  are  free  from  this  element. 
In  this  manner,  leucite,  nepheline,  and  the  feldspars  can  be 
extracted  from  the  bisilicate  rock-constituents ;  and  even  these 
latter  may  be  approximately  isolated  from  each  other,  by 
regulating  the  strength  of  the  current  according  to  the 
amount  of  iron  present  in  each. 

Any  notice,  however  brief,  of  the  origin  and  progress  of 
modern  petrography  would  be  very  incomplete  without  a 

*  Neues  Jahrbuch  fur  Mineralogie,  etc.,  1883,  II.,  p.  87. 
t  Memoires  de  1'Institut  de  France,  XII.,  No.  n. 
\  Neues  Jahrbuch  fur  Mineralogie,  etc.,  1881, 1.,  p.  172;  ibid.,  II  Beilage 
Band,  p.  620. 

§  Sitzungsberichte  der  Wiener  Akademie.     January,  1882. 


MODERN    PETROGRAPHY.  21 

reference  to  the  marvellous  results  recently  obtained  by  the 
French  investigators,  Fouque  and  Michel-Levy,  in  the  arti- 
ficial reproduction  of  volcanic  rocks.  In  their  laboratory  at 
the  College  de  France,  in  Paris,  these  savans  have,  within 
the  past  few  years,  succeeded,  by  using  the  simplest  appara- 
tus, in  synthetically  reproducing  nearly  all  the  basic  lavas  in 
such  perfection  that  even  a  microscopical  examination  fails 
to  disclose  any  essential  differences  between  their  productions 
and  those  of  nature.  This  has  been  accomplished  by  simply 
fusing  in  a  small  furnace*  either  the  powdered  minerals  of 
the  rock  to  be  formed,  or  of  the  ultimate  chemical  compounds, 
in  proper  proportions,  which  enter  into  its  composition.  Not 
only  were  the  natural  mineral  associations  which  are  charac- 
teristic of  the  basic  volcanic  rocks  in  this  way  obtained,  but 
it  was  found  that  even  the  different  kinds  of  rock-structure' 
—  glassy,  half  glassy,  porphyritic,  and  granular  —  could  also 
be  reproduced  by  regulating  the  temperature  in  various  ways. 
Crystallization  was  perfect  in  proportion  to  the  slowness  with 
which  the  mass  was  allowed  to  solidify.  The  minerals  gen- 
erally formed  in  the  order  of  their  fusibility,  so  that  individ- 
uals of  the  least  fusible  species,  like  leucite  and  olivine, 
could  be  obtained  imbedded  in  a  ground-mass,  which  could 
be  made  at  will  either  vitreous  or  a  finer  aggregate  of  the 
more  fusible  minerals.  A  temperature  just  below  the  fusing- 
point  of  any  mineral,  continued  for  from  two  to  three  days,  was 
found  sufficient  to  cause  this  to  assume  a  well  defined  crys- 
talline structure.  When  more  rapidly  cooled,  only  glass  was 
formed.  The  very  interesting  though  negative  result  was  also 
reached  that  minerals  especially  characteristic  of  the  acid 

*  The  furnace  (by  Forquignon  &  Leclercq)  and  blast  (by  Damoiseau) 
employed  in  these  experiments  are  admirably  figured  in  Fremy's  Encyclo- 
pedie  chimique,  II.,  App.  I.  Reproduction  artificielle  des  mineraux,  par 
M.  L.  Bourgeois,  PI.  I.  An  excellent  account  of  the  investigations  and 
their  results  is  also  given  in  the  same  place  (pp.  195-221). 


22  MODERN    PETROGRAPHY. 

rocks  —  such  as  quartz,  orthoclase,  hornblende,  mica,  etc. — 
could  not  be  formed  by  simple  fusion.  When  these  minerals 
were  employed,  they  invariably  passed  over  into  other  forms 
• — as,  for  instance,  hornblende,  which,  when  fused,  al\yays  re- 
crystallized  as  pyroxene.  Nature,  therefore,  probably  employs 
some  other  agency,  like  pressure  or  the  presence  of  water, 
not  attainable  in  the  laboratory,  for  the  production  of  her 
granites  and  rhyolites.  The  results  of  their  researches  in 
this  line  have  recently  been  described  at  length  by  Messrs. 
Fouque'  and  Michel-Levy  in  their  work  entitled  Synthese  des 
mineraux  et  des  roches* 

Such,  in  brief,  is  the  story  of  the  origin  and  develop- 
ment within  the  last  quarter  of  a  century  of  modern 
petrography.  Let  us  now  look  for  a  moment  at  its  rela- 
tionship to  other  allied  sciences  —  especially  at  its  bearing 
upon  geology,  of  which  it  may  most  properly  be  considered 
a  branch. 

We  have  seen  in  the  foregoing  sketch  how  the  microscope 
came  to  be  applied  to  the  study  of  inorganic  nature  at  first 
through  the  mere  desire  of  discovering  something  curious 
and  novel,  subsequently  on  account  of  the  new  field  of  sci- 
entific inquiry  which  its  almost  accidental  employment  sud- 
denly opened  up. 

It  was  altogether  necessary  that,  in  the  first  stages  of  its 
existence,  microscopical  petrography  should  be  cultivated  as 
an  extension  of  and  supplement  to  mineralogy.  To  the 
microscope,  mineralogy  already  owes  many  an  important  dis- 
covery. Through  its  agency  much  has  been  added  to  our 
knowledge  of  the  formation  of  crystals,  as  well  as  to  our 
understanding  of  their  internal  structure  and  impurities.  The 
same  means  has  also  thrown  light  upon  the  distribution  and 

*  Paris  :  1882, 8°,  pp.  423,  one  colored  plate  (Masson  ed.)-  Pages  i-So 
describe  especially  the  synthesis  of  rocks.  Vide,  also,  American  Chemi- 
cal Journal,  V.,  p.  127. 


MODERN    PETROGRAPHY.  2$ 

association  of  many  mineral  species  which  could  never  have 
been  secured  in  any  other  way.  With  regard,  finally,  to  the 
origin  of  certain  minerals,  almost  all  we  know  has  been 
derived  from  their  study  in  rock-sections.  There  is  no  rea- 
son to  believe  that  the  microscope  will  yield  less  to  miner- 
alogy in  the  future  than  it  has  in  the  past;  and  yet  it  is 
surely  not  by  its  pursuit  as  a  branch  of  mineralogy,  or  even 
as  an  end  in  itself,  that  modern  petrography  is  destined  to 
render  its  greatest  and  best  service  to  science. 

The  detailed  study  and  description  of  isolated  rock-speci- 
mens, which  heretofore  have  been  so  necessary  for  develop- 
ing the  methods  by  which  minerals  could  be  distinguished 
under  the  microscope,  will,  in  time  to  come,  have  but  little 
significance.  What  has  been  done,  and  so  well  done,  is  now 
accomplished  once  for  all. 

Most  accurate  and  delicate  methods  have  been  perfected 
by  years  of  patient  labor,  and  may  now  be  had  for  the  ask- 
ing. As  time  goes  on,  new  ones  will  be  added,  and  old  ones 
improved ;  and  yet  petrography  will  not  yield  her  best  service 
to  the  mineralogist,  but  to  the  geologist.  He  it  is  who  must 
thoroughly  master  the  new  and  potent  means  which  she 
places  at  his  disposal  for  successfully  dealing  with  many 
of  the  most  difficult  problems  presented  in  the  earth's 
crust. 

The  importance  of  what  the  microscope  could  do  for 
geology  has  not  been  unappreciated  in  the  past,  but  the  time 
has,  heretofore,  been  hardly  ripe  for  its  general  application 
in  this  field.  Geologists  are  now,  however,  the  heirs  of  the 
experience  and  the  results  which  many  years  of  slow  and 
patient  work  have  secured.  If  they  would  push  onward, 
they  must  apply  what  is  known  to  the  solution  of  what  is 
unknown.  The  microscope  must  now  become  as  necessary 
in  geology  as  it  is  in  zoology  and  botany.  Petrography  is 
daily  becoming  a  more  and  more  essential  part  of  the  train- 


24  MODERN    PETROGRAPHY. 

ing  of  every  general  geologist,  and  it  will  soon  be  regarded 
as  indispensable  as  mineralogy  or  palaeontology.  It  has  only 
to  be  understood  to  be  appreciated,  and  to  receive  a  place 
among  our  recognized  courses  of  instruction,  for  surely  there 
was  never  a  study  better  calculated  to  arouse  the  interest 
and  enthusiasm  of  those  who  delight  to  explore  the  hidden 
wonders  of  nature. 

The  application  of  the  microscope  to  questions  of  practi- 
cal importance  in  economic  geology,  —  as,  for  instance,  the 
study  of  building-stones  to  ascertain  their  strength  or  dura- 
bility, or  the  examination  of  rocks  for  ores  or  metals,  — 
notwithstanding  that  it  is  much  talked  of,  seems  at  present 
to  offer  but  little  that  is  encouraging ;  but  its  bearing  on  the 
deepest  questions  of  theoretical  geology  can  hardly  be  over- 
rated. It  now  appears  to  afford  almost  the  only  hopeful 
means  of  dealing  with  the  records  of  the  crystalline  strata  of 
the  earth,  which  undoubtedly  contain  the  longest,  as  they  do 
by  far  the  darkest,  chapter  of  its  history.  The  end  and  aim 
of  the  whole  science  of  geology  is  to  decipher  all  the  chap- 
ters of  this  history.  What  palaeontology  has  already  done 
and  is  still  doing  for  the  more  superficial  strata  in  which 
organic  remains  are  preserved,  the  microscope  must  do  for 
the  crystalline  rocks,  whether  volcanic,  plutonic,  or  meta- 
morphic.  These  contain  their  own  life-histories,  —  their 
origin  and  their  subsequent  alterations  under  the  action  of 
mechanical  and  chemical  forces, — written  in  characters 
which  need  only  to  be  carefully  studied  in  order  to  be  prop- 
erly interpreted.  And,  indeed,  much  has  already  been 
accomplished  in  this  direction.  The  nature  and  extent  of 
the  change  of  many  magnesian  silicates  to  serpentine  is  now 
well  known,  so  also  the  alteration  of  pyroxene  to  amphibole, 
of  orthoclase  to  muscovite,  and  of  many  rock-forming  miner- 
als to  chlorite,  epidote,  and  other  secondary  products.  Nor 
is  the  change  always  a  degeneration.  Rocks  often  become 


MODERN    PETROGRAPHY.  2$ 

more  crystalline  instead  of  less  so,  as  has  been  shown 
by  the  new  minerals,  like  quartz,  biotite,  garnet,  andalusite, 
etc.,  developed  in  clay-slates  near  their  contact  with  erup- 
tive rocks.  But  thus  far  only  enough  has  been  done  to 
merely  indicate  the  direction  in  which  the  most  promis- 
ing fields  for  work  lie.  The  far  greater  questions  of 
regional  metamorphism  and  the  origin  of  the  crystalline 
schists,  into  which  many  years  of  theorizing  have  suc- 
ceeded in  introducing  only  confusion  and  misconception, 
must  be  attacked  in  the  same  manner,  if  they  are  to  be 
solved  at  all. 

What  are  now  most  needed  are  careful  and  elaborate 
microscopical  studies  of  crystalline  rocks  in  close  connection 
with  an  equally  detailed  study  of  their  field-relations  —  the 
importance  and  necessity  of  which  has  been  increased,  not 
diminished,  by  the  laboratory  work.  As  already  stated,  the 
mere  microscopical  study  of  rocks  now  has  but  little  value. 
The  day  has  come  when  mountains  must  be  studied  with  the 
microscope,  but  not  with  the  microscope  alone.  Nor  is  it 
enough  that  the  field-geologist  should  intrust  the  microscopical 
examination  of  his  works  to  an  expert ;  each  should  be  so 
far  a  master  of  the  methods  of  petrography  and  of  what  has 
already  been  accomplished  in  it,  as  to  be  able  to  make  his 
own  observations,  and  to  intelligently  appreciate  the  results 
obtained  by  others.  The  great  value  of  such  a  combination 
of  field  and  laboratory  work  for  the  elucidation  of  the 
enigmas  of  regional  metamorphism  could  have  no  better 
proof  than  the  recent  labors  of  Lossen  *  in  the  Hartz  Moun- 
tains, and  of  Lehmann  f  in  Saxony.  They  have  succeeded 

*  Studien  an  metamorphischen  Eruptiv-  und  Sedimentgesteinen. 
Jahrbuch  der  k.  preuss.  geol.  Landesanstalt  fur  1883,  p.  619;  ibid.,  1884, 

P-  525- 

t  Die  Entstehung  der  altkrystallinen  Schiefergesteine,  etc.    4°  Bonn : 

1884. 


26  MODERN    PETROGRAPHY. 

in  showing  how  eruptive  as  well  as  sedimentary  rocks  undergo 
alteration  in  structure,  as  well  as  mineralogical  composition, 
when  subjected  to  the  great  compression  or  strains  attendant 
on  the  elevation  of  mountains.  Their  work,  which  has  only 
to  be  thoroughly  understood  in  order  that  its  value  should  be 
fully  appreciated,  offers  an  inspiring  example  to  all  earnest 
students  of  geology  the  world  over. 


BIBLIOGRAPHY. 


COMPLETE  bibliographies  of  petrography  are  given  in  the 
larger  works  of  Rosenbusch  and  Fouque'  and  Michel-Levy. 
It  is  intended  to  mention  here  only  the  titles  of  such  impor- 
tant books  and  papers  as  may  be  of  service  to  those  who  . 
wish  to  provide  themselves  with  the  most  necessary  petro- 
graphical  literature. 

I.   On  the  microscopical  determination  of  minerals  in  rock- 
sections. 

1869-1873.     H.  FISCHER:  Kritische  mikroskopisch-mineralo- 

gische  Studien.     3  nos.     Freiburg. 
1873.     F.  ZIRKEL  :    Die  mikroskopische  Beschaffenheit  der 

Mineralien  und  Gesteine.     Leipzig :  8°  502  pp. 
1873.     H.  ROSENBUSCH  :  Die  mikroskopische  Physiographic 

der  petrographisch  wichtigen  Mineralien.     Stuttgart:  8° 

398  pp.,  ten  colored  plates. 

1885.     Same,  second  edition,  644  pp.,  and  twenty-six  photo- 
graphic plates. 
1876.     C.  DOELTER  :    Die  Bestimmung   der  petrographisch 

wichtigen  Mineralien  durch  das  Mikroskop.     Vienna :  8° 

36  pp.    (Unimportant.) 

1879.  F.  FOUQUE  et  A.  MICHEL-LEVY:   Mineralogie  micro- 
graphique  des  roches  eruptives  franchises.    Paris  :  4°  508 
pp.,  and  an  Atlas  of  fifty-five  plates. 

1880.  J.  THOULET  :  Contributions  a  1'e'tude  des  proprie'te's 
physiques    et   chimiques    des    mineraux   microscopiques. 
Theses  pre'sente'e  &  la  Faculte'  des  sciences  de  Paris.    Paris. 

27 


28  MODERN    PETROGRAPHY. 

1885.  E.  HUSSAK:  Anleitung  zum  Bestimmen  der  gesteins- 
bildenden  Mineralien.      Leipzig :  8°  196  pp. 

1886.  Same,  translated  by  E.  G.  SMITH  as  Determination 
of  Rock-forming  Minerals.     New  York  :  8°  233  pp. 

A  complete  bibliography  of  the  methods  employed  in  mod- 
ern petrographical  investigations  is  contained  in  E.  COHEN'S 
Zusammerfstellungpetrographischer  Untersuch ungsmethoden,  pri- 
vately printed.  Strasburg,  March  4,  1884 :  8°  19  pp. 


2.  General  descriptive  works. 
1860.     R.  BLUM  :  Handbuch  der  Lithologie  oder  Gesteins- 

lere.     Erlangen  :  8°  356  pp. 
1866.     B.  VON  COTTA  :  Rocks  classified  and  described.     A 

treatise   on   Lithology,  translated   by   P.    H.   Lawrence. 

London :  8°  425. 

1866.  F.  ZIRKEL  :    Lehrbuch  der  Petrographie.     Bonn  :  8° 
two  vols.,  607,  635  pp. 

[None  of  these  three  works  contains  any  allusion  to  the 
microscope.] 

1867.  H.    VOGELSANG:     Philosophic    der    Geologic    und 
mikroskopische  Gesteinsstudien.     Bonn  :  8°  229  pp.,  and 
ten  colored  plates. 

1869.  G.  TSCHERMAK:    Die   Porphyrgesteine   Oesterreichs 
aus   der   mittleren   geologischen   Epoche.      Vienna  :    8° 
281  pp. 

1870.  F.   ZIRKEL  :    Untersuchungen   iiber   die   mikroskop- 
ische Zusammensetzung  und  Structur  der  Basaltgesteine. 
Bonn  :  8°  208  pp.,  and  three  plates. 

1871.  R.   HAGGE  :    Mikroskopische   Untersuchungen    iiber 
Gabbro  und  verwandte  Gesteine.     Kiel :  8°  63  pp. 

1873.     F.  ZIRKEL  :  Mikroskopische  Beschaffenheit  der  Min- 
eralien und  Gesteine.     Leipzig:  8°  pp.  265-475. 


BIBLIOGRAPHY.  2Q 

1875.     A.  VON  LASAULX  :  Elemente  der  Petrographie.    Bonn. 

1875.  H.VOGELSANG:  Die  Krystalliten  —  nach  dem  Tode 
des  Verfassers  herausgegeben  von  F.  Zirkel.     Bonn  :  8° 
175  pp.,  and  sixteen  colored  plates. 

1876.  F.  ZIRKEL  :   Microscopical  Petrography,  Vol.  VI.  of 
the  Report  of  the  Geological  Exploration  of  the  Fortieth 
Parallel.     Washington:   4°  297  pp.,  and   twelve  colored 
plates. 

1877.  H.  ROSENBUSCH  :  Mikroskopische  Physiographic  der 
massigen  Gesteine.     Stuttgart:    8°  596  pp.     (A  second 
edition   of   this   most  important  work   is   announced  for 
August,  1886.) 

1877.  O.LANG:  Grundriss  der  Gesteinskunde.    Leipzig:  8°. 

1878.  G.  W.  HAWES:    Mineralogy   and  Lithology  of   New 
Hampshire.     Part  IV.  of   the  Geology  of  New  Hamp- 
shire, by  C.  H.  Hitchcock.     Concord:   4°  251    pp.  and 
twelve  colored  plates. 

1879.  F.  RUTLEY  :  The  Study  of  Rocks.     London. 

1 88 1.  A.  COSSA  :  Ricerche  chimiche  e  microscopiche  su 
roccie  e  minerali  d'ltalia  (1875-1880).  Turin:  4°  502 
pp.,  and  twelve  colored  plates. 

1883.  E.  COHEN  :  Sammlung  von  Mikrophotographien  zur 
Veranschaulichung  der  mikroskopischen  Structur  von 
Mineralien  and  Gesteinen.  Stuttgart:  4°  —  480  micro- 
photographs  in  ten  portfolios. 

1883.  J.  ROTH  :   Allgemeine  chemische   Geologic,  vol.  II. 
Petrographie.     Berlin  :  8°.     (Not  yet  completed.) 

1884.  K.  W.  von  GUMBEL  :  Geologic  von  Bayern,  I  Theil. 
Cassel :  8°  (not  yet  completed).    Petrography,  pp.  1-211. 

1884.     ED.  JANNETAZ  :  Les  roches.     Paris. 

1884.      M.    E.  WADSWORTH  :    Lithological  Studies,  Part  I. 

Cambridge  :  4°,  with  eight  colored  plates. 
1886.     A.  VON  LASAULX:  Einfuhrung  in  die  Gesteinslehre. 

Breslau  :  8°  215  pp. 


3O  MODERN    PETROGRAPHY. 

1886.     E.  KALKOWSKY:  Elemente  der  Lithologie.     Heidel- 
berg :  8°  316  pp. 


j>.   On  the  chemical  analyses  of  rocks. 
1861.     J.  ROTH  :  Gesteinsanalysen  in  tabellarischer  Ueber- 

sicht  und  mit  kritischen  Erlauterungen.     Berlin  :  4°. 
1869.      J.    ROTH  :    Beitrage    zur   Petrographie   der  pluton- 

ischen  Gesteine.     Abhandlung  der  k.  Akad.  der  Wissen- 

schaft.     Berlin  :  4°. 
1873.     J.  ROTH  :  do.     Berlin. 
1879.     J.  ROTH  :  do.     Berlin. 
1884.     J.  ROTH  :  do.     Berlin. 


4.   On  the  artificial  reproduction  of  minerals  and  rocks. 
1857.     GURLT:    Uebersicht   der    pyrogenneten   kiinstlichen 

Mineralien.     Freiberg. 
1872.     FUCHS:    Die     kiinstlich     dargestellten     Mineralien. 

Haarlem  :  4°. 
1879.     A.  DAUBREE  :  Etudes  synthe'tiques  de   Ge'ologie  Ex- 

pe'rimentale.     Paris :  8°. 
1882.     F.  FOUQUE  et  A.  MICHEL-LEVY  :  Synthese  des  mine- 

raux  et  des  roches.     Paris  :  8°  423  pp. 
1884.     M.  L.  BOURGEOIS  :  Reproduction  artificielle  des  mine- 

raux.     (Appendix  I.  to  Vol.  II.  of  Fre'my's  Encyclopedic 

chimique.)     Paris  :  8°  240  pp.  and  eight  plates. 


The  most  important  periodicals  in  which  petrographical 
papers  have  been  or  are  still  published  are  the  follow- 
ing:— 


BIBLIOGRAPHY.  3! 

Neues  Jahrbuch  fur  Miner alogie,  Geologic,  und  Palceontologie. 
Published  at  Stuttgart  in  two  yearly  volumes  of  three 
numbers  each.  This  contains  valuable  reviews  of  all  the 
geological  literature  of  the  world.  "  Beilage  Bande  "  are 
also  issued  from  time  to  time,  to  contain  the  surplus  of 
original  papers. 

Mineralogische  und  Petrographische  Mittheilungen,  herausgege- 
ben  von  Gustav  Tschermak.  Vienna :  8°,  one  volume 
yearly. 

Jahrbuch  der  k.  k.  geologischen  Reichsanstalt.  Vienna  :  4°,  one 
volume  yearly. 

Zeitschrift  der  deutschen  geologischen  Gesellschaft.     Berlin  :  8°. 

Zeitschrift  fur  Krystallographie  und  Mineralogie,  heraus- 
gegeben  von  P.  Groth.  Leipzig :  8°. 

Jahrbuch  der  kb'nigL  preuss.  geologischen  Landesanstalt.  Ber- 
lin :  4°. 

Poggendorff's  Annalen  fur  Physik  und  Chemie.     Leipzig  :  8°. 

Sitzungsberichte  der  K.  K.  Akademie  der  Wissenschaften  zu 
Wien.  8°. 

Bulletin  de  la  Societe  geologique  de  France.     Paris  :  8°. 

Bulletin  de  la  Socie'te  mineralogique  de  France.     Paris  :  8°. 

Annales  des  Mines.     Paris  :  8°. 

Comptes  rendus  hebdomadaires  de  ^Academic  fran$aise. 
Paris :  4°. 

Annales  de  la  Societe  geologique  du  Nord.     Lille  :  8°. 

Quarterly  Journal  of  the  Geological  Society.     London  :  8°. 

The  Geological  Magazine.     London  :  8°. 

The  Mineralogical  Magazine.     London  :  8°. 

The  American  Journal  of  Science  and  Arts.  New  Haven  :  8°, 
two  volumes  yearly. 

The  Bulletins  of  the  U.  S.  Geological  Survey.    Washington  :  8°. 

Geoligiska  Foreningens  i  Stockholm  Forhandlingar.  Stock- 
holm:  8°. 

Nyt  Magazinfor  Naturvidenskaberne.     Christiania :  8°. 


32  MODERN    PETROGRAPHY. 


NOTE  ON  THE  PREPARATION  OF  ROCK-SEC- 
TIONS AND  ON  THE  MANUFACTURERS  OF 
PETROGRAPHICAL  APPARATUS,  ETC. 

EXTENSIVE  and  admirable  directions  for  the  preparation  of 
thin-sections  of  rocks  for  the  microscope  will  be  found  in 
the  above-cited  works  of  Rosenbusch  and  Hussak  (see  Bibli- 
ography). The  process  is  by  no  means  as  difficult  or  tedious 
as  at  first  might  appear,  or,  indeed,  as  it  is  described  by 
Rutley  and  some  other  authors.  As  thin  a  chip  as  possible 
is  knocked  off  with  a  hammer  from  the  rock-specimen  to  be 
investigated.  This  should  not  be  over  half  an  inch  in 
diameter.  One  side  of  this  is  now  ground  down  with  emery 
(about  No.  70)  upon  an  iron  or  copper  plate.  It  is  conve- 
nient to  have  a  revolving  disc  for  this  purpose ;  but  grinding 
with  the  free  hand  upon  a  stationary  plate  is  easily  accom- 
plished, and  does  not  consume  much  more  time.  When  the 
chip  is  approximately  even  on  one  side,  it  is  rubbed  on 
another  plate  with  finer  emery,  and  finally  upon  a  piece  of 
plate-glass  with  emery-flour.  It  does  not  need  to  be  pol- 
ished, as  all  unevenness  disappears  when  it  is  imbedded  in 
Canada  balsam.  The  chip  must  now  be  thoroughly  washed 
and  dried,  and  then  mounted  on  a  small  rectangular  piece  of 
glass.  [It  is  most  convenient  to  have  a  number  of  pieces  of 
ordinary  window-glass  cut  by  a  glazier  into  the  proper  size 
(i|Xi  inch)  for  this  purpose.]  This  glass  must  be  heated 
over  a  small  flame  of  a  Bunsen  burner,  and  then  the  .balsam 
placed  upon  it.  This  must  now  be  heated  (without  being 
allowed  to  catch  fire)  until  enough  of  the  volatile  portion  has 
been  driven  off  to  leave  the  balsam  hard  on  cooling.  Into 
the  hot  balsam  the  rock,  after  being  dried  over  the  flame,  is 
pressed,  and  allowed  to  cool,  care  being  taken  that  no  bub- 


NOTE.  33 

bles  of  air  remain  between  it  and  the  glass.  When  quite 
cool,  the  balsam  should  yield  only  a  little  to  the  pressure  of 
the  nail,  but  not  be  brittle  enough  to  break  away.  Some 
practical  experience  is  necessary  to  secure  just  the  right 
consistency.  A  better  plan  is  to  harden  a  considerable  quan- 
tity of  balsam  at  one  time  so  as  to  have  it  ready  for  con- 
stant use,  when  it  is  merely  softened  by  heat 

After  it  has  been  satisfactorily  njounted,  the  chip  is  ground 
down  with  emery  as  before  until  only  a  fine  film  of  rock 
remains  on  the  glass.  It  should  be  examined  from  time  to 
time  under  the  microscope,  being  temporarily  covered  with 
a  drop  of  water  and  a  cover-glass. 

When  the  requisite  thinness  is  reached,  the  section  is 
mounted  in  its  final  shape.  For  this  purpose  it  is  carefully 
washed  and  the  superfluous  balsam  scraped  from  around  its 
edges  with  a  knife.  A  drop  of  the  best  quality  of  balsam  is 
now  placed  upon  the  mounting-glass,  and  another  upon  the 
section.  The  former  is  heated  until  it  will  harden  on 
cooling ;  the  latter  until  the  balsam  by  which  it  adheres  to 
the  first  glass  is  so  far  softened  as  to  allow  of  the  section 
being  gently  pushed  with  a  needle  over  on  to  the  mounting- 
glass.  When  this  is  accomplished,  a  cover-glass  is  quickly 
laid  upon  it  and  pressed  down  till  all  bubbles  of  air  are  forced 
out.  Then  the  whole  is  allowed  to  cool,  and  the  superfluous 
balsam  washed  from  the  glass  with  alchohol.  The  best 
shaped  glass  for  mounting  rock-sections  is  square.*  Long 
glasses  seriously  interfere  with  the  constant  revolution  of  the 
microscope  stage  necessary  in  studying  the  sections.  The 
locality  and  number  of  the  specimen  may  advantageously  be 
written  on  this  glass  with  a  diamond. 

A  great  saving  of  time  in  the  preparation  of  rock-sections 

*  Excellent  glasses  for  mounting,  32  mm.  square,  may  be  obtained 
from  P.  Stender,  No.  1 1  Konigstrasse,  Leipzig,  Germany.  They  are  not 
yet,  as  far  as  I  know,  to  be  had  in  this  country. 


34  MODERN    PETROGRAPHY. 

is  secured  by  the  employment  of  a  rapidly  revolving  tin  disc 
whose  edge  is  prepared  with  diamond  dust.*  Such  a  disc 
may  be  mounted  like  a  buzz-saw  and,  when  only  moistened 
with  water,  will  be  found  to  cut  through  the  hardest  rocks 
with  a  surprising  rapidity.  A  fragment  of  rock  is  first  cut  in 
two  and  its  flat  side  smoothed  down  with  emery-flour  on  a 
glass  plate.  It  is  then  cemented  to  a  glass,  as  above  des- 
cribed, and  again  sawed  ^ff  close  to  this.  A  section  may 
thus  be  obtained  which  is  translucent,  and  which  needs 
but  little  further  grinding  to  render  it  fit  for  microscopic 
study. 

No  thoroughly  satisfactory  petrographical  microscope  has 
yet  been  manufactured  in  the  United  States,  although  it  is 
hoped  that  this  want  may  soon  be  supplied.  The  best  cheap 
instruments  come  from  Germany,  and  may  be  had  with  all 
needful  accessories  for  about  $100.00.  The  firms  of 
£.  Fuess,  1 08  Alte  Jacob-strasse,  Berlin,  S.  W.,  and  Voigt 
and  Hochgesang,  Gottingen,  are  to  be  especially  recom- 
mended. Either  one  will  furnish  a  serviceable  instrument, 
constructed  on  the  old  Rosenbusch  model, f  at  the  price 
above  mentioned.  Much  larger  and  more  complete  instru- 
ments are  made  by  both  of  these  firms,  and  supplied  with 
every  conceivable  device  to  aid  in  petrographical  research. 
That  by  Fuess  costs  500  marks  ($125.00)  without  objectives 
or  spectroscopic  apparatus, \  while  the  larger  instrument  of 
Voigt  and  Hochgesang  is  priced  at  800  marks  ($200.00) 

*  Discs  so  prepared  may  be  obtained  of  Wm.  Kerr,  No.  35  West- 
minster St.,  Providence,  R.  I.  They  are  in  use  by  the  U.  S.  Geological 
Survey  and  in  the  Petrographical  Laboratory  at  the  Johns  Hopkins 
University,  where  they  give  great  satisfaction.  They  may  be  had  of  any 
size,  and  cost  eighty-seven  cents  for  each  inch  of  their  diameter. 

t  See  Neues  Jahrbuch  fur  Mineralogie,  etc.,  1876. 

%  See  Rosenbusch:  Petrographisch  wichtige  Mineralien.  2d  ed., 
p.  562.  This  is  fully  described  by  Th,  Liebisch :  Neues  Jahrbuch  fiir 
Mineralogie,  etc.,  1886,  II. 


NOTE.  /* V;,,  ;  ; '  J>,  /  ; -? ;  vj  >  //»35 

complete.*  Full  descriptive  catalogues  will  be  sent  by 
either  firm  on  application. 

Undoubtedly  the  best  large  microscope  for  petrographical 
work  is  made  by  the  firm  of  Nachet  et  Fils,  17  Rue  St.  Severin, 
Paris.  The  advantage  of  the  Nachet  model  is  that  the  tube 
is  divided,  and  the  objective  made  to  revolve  together  with 
the  stage.  This  secures  a  perfectly  concentric  adjustment, 
the  value  of  which  is  apparent  to  all  who  have  had  any  ex- 
perience with  petrographical  microscopes.t  The  entire  cost 
of  the  large  Nachet  instrument  is  1200  francs  ($250.00). 

Rock-sections  of  typical  material  from  well  known  Euro- 
pean localities,  put  up  in  collections,  may  be  obtained  from 
either  of  the  above-mentioned  German  firms.  They  will 
also  prepare  either  rock  or  crystal  sections  to  order  at  an 
average  price  of  i  mark  (25  cents)  apiece. 

Rock-sections  are  prepared  in  this  country  by  G.  D.  Julien, 
corner  Fiftieth  Street  and  Fourth  Avenue,  New  York,  and  by 
Queen  &  Co.  of  Philadelphia.  The  prices,  however,  of  both 
these  firms  are  very  high.  Sections  may  be  most  advan- 
tageously obtained  of  Mr.  Hermann  Ohm,  care  of  the  U.  S. 
Geological  Survey,  Washington,  D.  C.,  at  the  cost  of  $0.40 
each.  From  a  personal  experience,  the  writer  can  recom- 
mend these  sections  as  the  finest  he  has  ever  seen. 

The  separating-funnel,  described  on  page  17,  for  separat- 
ing rock-constituents  by  means  of  a  high  specific  gravity 
solution  have  been  made  for  some  years  past  by  Karl 
Kramer,  Freiburg  in  Baden,  Germany. 

The  borotungstate  of  cadmium  solution  (Klein's  solution) 
may  be  obtained  of  E.  Rousseau  et  Fils,  42  &  44  Rue  des 
Ecoles,  Paris,  who  manufacture  it  for  petrographical  purposes. 

*  This  microscope  has  been  fully  described  by  Prof.   C.  Klein  in 
the  Neues  Jahrbuch  fur  Mineralogie,  etc.,  Ill  Beilage  Band,  p.  540. 
t  See  Rosenbusch:  loc.  cit., 


SCIENCE. 


First  Book  in  Geology. 


By  N.  S.  SHALER,  Professor  of  Paleontology,  Harvard  University.  5^  by 
7^  inches.  Cloth.  xvii+  255  pages,  with  130  figures  in  the  text.  74  pages 
additional  in  Teacher's  Edition.  Price  by  mail,  $1.10;  Introduction,  $1.00. 

'"PHE  design  of  this  book  is  to  give  the  student  from  ten  to  fifteen 
years  of  age  a  few,  clear,  well-selected  facts  that  may  serve  as  a 
key  to  the  knowledge  of  the  earth.  The  number  of  facts  dealt  with 
is  far  less  than  is  usually  given  in  such  books,  but  pains  is  taken  in 
their  presentations  to  make  them  open  the  way  to  the  broadest  veins 
that  the  science  affords.  The  aim  is  to  illustrate  the  principles  of 
geology  by  reference  to  as  many  facts  of  familiar  experience  as  pos- 
sible. 

The  first  part  of  the  book  treats  of  the  simpler  phenomena  of  a 
physical  sort,  the  movements  of  the  water  and  the  air,  and  their  effect 
on  the  machinery  of  the  earth's  surface ;  then  the  simpler  underground 
actions  are  taken  up,  such  as  the  formation  of  veins,  the  folding  of 
mountains,  and  the  forces  that  lead  to  earthquakes  and  volcanoes. 
The  latter  half  of  the  book  is  given  to  the  history,  in  outline,  of  the 
earth's  organic  life.  This  is  treated  in  a  very  general  way,  in  order  to 
show  the  student  only  the  great  steps  of  advance,  and  the  method  in 
which  they  are  accomplished. 

In  the  appendix  is  a  brief  account  of  certain  more  important  mineral 
species,  arranged  to  give  the  student  an  outline  of  mineralogy,  and 
some  idea  of  the  common  uses  of  minerals. 

The  Teacher's  Edition  contains  seventy-four  pages  of  directions  for 
those  who  use  the  book  in  class  instruction.  First  there  are  general 
directions  for  the  guidance  of  teachers  in  their  work  in  natural  history, 
then  each  chapter  of  the  book  is  taken  up  in  turn,  and  the  instructor  is 
told  how  to  supplement  each  lesson,  by  reference  to  facts  that  may  be 
easily  accessible  in  the  nature  about  the  school. 

The  instructor  who  will  make  proper  use  of  these  pages  will  always 
find  it  possible  to  enliven  the  printed  page  with  many  an  illustration  of 
value  to  his  students.  And  the  average  reader  who  desires  to  get  a 
glance  at  geology  and  a  general  notion  of  its  bearings  on  ordinary  life, 
will  find  this  edition  of  exceeding  interest.  It  is  being  used  in  many 
schools  as  a  Supplementary  Reader,  and  is  admirably  adapted  for  such 
purpose. 


SCIENCE. 


The  following  letters  show  how  the  book  has  been  re- 
ceived by  Teachers  of  Geology  :  — 


Alexander  Winchell,  Chair  of 
Geology  and  Paleontology,  University  of 
Mich.  :  I  have  looked  it  through  with 
extraordinary  interest;  for  its  marked 
departure  from  the  methods  of  the  old 
didactic  treatises  which  have  done  so 
much  to  put  geology  at  a  disadvantage, 
in  comparison  with  botany,  is  in  the  di- 
rection of  common  sense  and  in  the 
interests  of  science  and  education.  I 
believe  the  book  will  have,  as  it  deserves, 
an  extensive  use  in  the  schools. 
(July  23,  1884.) 

Albert  A.  Wright,  Prof,  of  Geology 
and  Nat.  Hist.,  Oberlin  Coll. :  I  think  it 
is  admirably  adapted  to  its  purpose,  both 
in  matter  and  style.  I  have  examined 
only  parts  of  it,  but  am  pleased  with  the 
treatment  at  every  point. 
(July  8,  1884.) 

Jas.  M.  Safford,  Prof,  of  Geology, 
Vanderbilt  Univ. :  I  am  prepared  to  en- 
dorse it  for  the  purpose  and  persons  in- 
tended. Prof.  Shaler  has  treated  the 
subject-matter  well  and  satisfactorily. 
(Sept.  27,  1884.) 

Edward  M.  Shepard,  Prof,  of  Nat- 
ural Science,  Drury  Coll.,  Springfield, 
Mo.  :  It  is  by  far  the  best  book  of  the 
kind  that  I  have  seen.  (Sept.  17,  1884.) 

David  S.  Jordan,  Pres.  of  Indiana 
Univ.  :  From  my  acquaintance  with  the 
author,  I  have  no  doubt  it  will  prove 
thoroughly  satisfactory.  It  is  not  unlikely 
that  I  may  use  it.  (Oct.  4,  1884.) 

John  C.  Branner,  Prof,  of  Geology, 
Indiana  Univ. :  With  a  view  to  urg- 
ing the  use  of  some  elementary  book 
on  geology  in  the  schools,  of  this  State, 
I  have  examined  Prof.  Shaler's  First 
Book  in  Geology.  I  cannot  do  better 
than  recommend  it.  The  limited  num- 


ber of  subjects  treated  saves  the  book 
from  the  charges  brought  against  other 
elementary  books  which  try  to  teach  too 
much  in  a  short  time  to  pupils  not  pre- 
pared to  receive  it.  The  advice  to 
teachers  is  also  very  much  to  the  point, 
and  will  doubtless  be  thoroughly  appre- 
ciated. (6)^.26,1885.) 

Henry  L.  Osborn,  Prof,  of  Geology, 
Purdue  Univ. :  I  am  very  favorably  im- 
pressed with  the  character  of  the  work, 
and  shall  take  every  opportunity  of  rec- 
ommending its  use.  (Dec.  3,  1885.) 

H.  A.  Huston,  Instructor  in  Physics, 
Purdue  Univ.,  Lafayette,  Ind. :  I  had 
the  pleasure  of  recommending  the  book 
to  several  Eastern  teachers  during  the 
summer,  and  shall  call  the  attention  of 
my  successor  in  the  high  school  here  to 
the  work.  (Sept.  28,  1884.) 

Herbert  Osborn,  Prof,  of  Ento- 
mology, Iowa  Agric.  Coll.:  I  am  very 
much  pleased  with  its  plan,  and  with 
the  contents  in  general,  and  deem  it  well 
worthy  of  high  commendation. 
(Nov.  19,  1884.) 

P.  A.  Chase,  Dep't  of  Physical  Sci- 
ence, Fisk  Univ.  :  It  is  written  in  a  most 
interesting  style,  and  is  much  more  suc- 
cessful in  adapting  geological  instruction 
to  immature  minds  than  most  works  of 
the  class.  (0^.11,1884.) 

Win.  A.  Obenchain,  Pres.  Ogden 
Coll.,  Ky.:  I  am  really  charmed  with 
Shaler's  First  Book  in  Geology.  Most 
heartily  do  I  commend  it  to  the  high 
schools  of  Kentucky. 

L.    M.    Underwood,   Instructor  in 

Geology,  Syracuse  Univ.  :  It  commends 
itself  to  general  use  on  account  of  the 
simplicity  with  which  great  truths  are 


SCIENCE. 


Illustrations  of  Geology  and  Geography. 

For  Use  in  Schools  and  Families.  By  N.  S.  SHALER,  Professor  of  Palaeon- 
tology, assisted  by  WM.  M.  DAVIS,  Assistant  Professor  of  Physical  Geography, 
and  T.  \V.  HARRIS,  Assistant  in  Botany,  in  Harvard  University. 

CONSISTING  of  twenty  large  photographs  and  an  equal  number  of 
^-'  colored  plaster  models.  The  photographs  are  separately  mounted 
on  suitable  light  frames,  15x20  inches  in  size.  They  represent  a  wide 
range  of  terrestrial  phenomena,  seashores,  valleys,  glaciers,  mountains, 
volcanoes,  caverns,  etc.  Alongside  of  each  photograph  is  a  detailed 
description  of  the  important  points  illustrated  in  the  picture,  with 
occasional  small  diagrams,  designed  to  show  the  detailed  structure  of 
the  field ;  also  references  to  the  features  in  the  models,  which  serve  to 
explain  the  facts  shown  in  the  view. 

The  models,  which  are  colored,  are  each  7x5  inches,  and  about 
2  inches  thick.  One  series  shows  the  principal  features  of  horizontal, 
tilted,  and  folded  stratified  rocks,  and  the  varied  effects  of  river  and 
ocean  erosion  upon  them ;  others  exhibit  the  process  of  development 
of  a  volcano,  of  coral  islands,  of  ocean  shores,  glaciers,  etc.  These 
models  are  separately  mounted  on  wooden  backs,  to  which  are  appended 
descriptions  of  the  structures  indicated,  with  reference  to  the  photo- 
graphs. 

In  the  text  appended  to  both  models  and  photographs,  there  are 
abundant  references  to  several  text-books,  where  further  information 
may  be  obtained.  They  are  large  enough  to  be  seen,  when  in  the  in- 
structor's hand,  by  a  class  of  thirty  students.  They  are  designed  to 
hang  on  the  wall,  and  may,  when  necessary,  be  passed  from  hand  to 
hand  without  injury. 

The  price  of  the  full  collection  of  fifty  pieces,  securely  boxed  for 
transportation,  is  one  hundred  dollars.  A  smaller  set,  containing  ten 
models  and  ten  photographs,  will  be  sold  at  fifty  dollars.  When  desired, 
the  collection  will  be  divided,  and  the  models  or  photographs  sold  sep- 
arately ;  the  price  for  each  set  of  twenty-five  pieces  will  be  fifty  dollars. 
Specimen  copies  of  the  models  and  photographs,  one  of  each,  to  show 
the  nature  of  the  method,  will  be  sent  by  express,  carriage  paid,  on 
receipt  of  four  dollars,  which  will  be  returned  on  the  receipt  of  the 
objects  in  good  order,  or  accounted  for  if  the  collection  is  taken.  A 
circular  containing  a  detailed  list  of  the  models  and  photographs  will  be 
sent  on  application.  [Ready  Aug.  i. 


SCIENCE. 


Guides  for  Science  Teaching. 


Published  under  the  auspices  of  the  Boston  Society  of  Natural 
History. 

TNTENDED  for  the  use  of  teachers  who  desire  to  practically  instruct 
-*-  their  classes  in  Natural  History,  and  designed  to  supply  such  infor- 
mation as  they  need  in  teaching  and  are  not  likely  to  get  from  any 
other  source. 

These  Guides  were  prepared  solely  as  aids  to  teachers,  —  not  as  text- 
books. The  plan  of  teaching  followed  throughout  is  based  upon  the 
assumption  that,  — 

Seeing  is  the  first  step  on  the  road  to  knowledge ;  that,  — 

How  MUCH  the  child  learns  in  his  early  years  is  of  little  importance, 
—  HOW  he  learns,  everything',  that,  — 

The  teacher's  work  is  not  to  teach  the  facts,  but  to  lead  the  mind  of 
each  pupil  to  work  out  for  itself  the  simple  physical  problems  witnessed 
or  described,  and  to  cultivate  the  habit  of  observation  arid  of  persever- 
ance in  investigation. 

The  Series  at  present  consist  of  the  following  numbers :  — 

About  Pebbles.    (No.  I.) 

By  ALPHEUS  HYATT,  Professor  of  Zoology  and  Paleontology  in  the  Massa- 
chusetts Institute  of  Technology.  4^  by  6  inches.  Paper.  26  pages. 
Introduction  price,  10  cents. 

This  pamphlet  is  an  illustration  of  the  way  in  which  a  few  common 
objects  may  be  used  to  cultivate  the  powers  of  observation,  and  to 
teach  interesting  lessons  in  elementary  natural  science.  It  contains  all 
the  suggestions  necessary  to  enable  any  teacher  to  make  the  lesson,  or 
lessons,  a  complete  success. 

Concerning  a  Few  Common  Plants.      (No.  II.) 

By  GEORGE  LINCOLN  GOODALE,  Professor  of  Botany  in  Harvard  Univer- 
sity. 4^  by  6  inches.  Paper.  61  pages.  Introduction  price,  10  cents. 

The  design  of  these  lessons  is  to  point  out  one  method  by  which  a 
few  of  the  more  important  and  easily  observed  facts  can  be  taught 
respecting  the  structure,  growth,  and  work  of  plants.  The  purpose 
of  this  Guide  is  to  call  attention  to  the  manner  of  preparing  the 


SCIENCE. 


A  Set  of  Fifteen  Specimens,  to  be  used  in  connection  with 
Guide  VII.,  will  be  furnished  for  $1.00. 

Orders  for  Specimens  to  accompany  Guides  III.,  IV.,  V.,  VI.,  or 
VII.,  should  be  addressed  to  SAMUEL  HENSHAW,  Boston  Society  of 
Natural  History,  Boston,  Mass. 

Larger  collections,  and  sets  for  students1  use,  containing  ten,  twenty, 
forty,  and  sixty  specimens  of  a  single  form,  can  be  obtained  by  special 
arrangement  with  Mr.  Henshaw. 

Common  Minerals  and  Rocks.     (No.  XII.) 

By  W.  O.  CROSBY,  Assistant  Professor  of  Mineralogy  and  Lithology  in  the 
Massachusetts  Institute  of  Technology.  Illustrated.  4^  by  6  inches. 
Paper.  200  pages.  Introduction  price,  40  cents.  Cloth,  60  cents. 

This  includes,  first,  a  brief  and  simple  account  of  the  principal  geo- 
logical agencies ;  second,  descriptions  of  about  twenty  minerals  of  which 
rocks  are  chiefly  composed,  and  of  all  the  more  common  and  important 
varieties  of  rocks ;  and,  third,  an  explanation  of  the  leading  kinds  of 
structure  occurring  in  rocks,  such  as  stratification,  folds,  faults,  joints, 
etc.  This  last  section  of  the  Guide  is  illustrated  by  forty  figures,  which 
add  very  materially  to  the  clearness  and  value  of  the  text. 

Especial  prominence  is  given  to  the  easy  identification  of  the  com- 
mon minerals  and  rocks,  and  to  the  constant  association,  in  the  mind, 
of  the  rocks  and  rock-structures  with  the  agencies  by  which  they  have 
been  formed. 

This  little  volume  is  not  merely  a  guide  to  teachers,  but  it  is  also  a 
simple  and  logical  presentation  of  the  leading  facts  and  principles  of 
structural  geology,  and  is  well  adapted  for  class  use.  It  is  hoped,  how- 
ever, that  teachers  will  base  their  instruction  upon  specimens  of  min- 
erals and  rocks,  using  this  work  more  as  a  reference  book  than  as  a 
text-book,  in  the  hands  of  pupils.  Natural  science  cannot  be  success- 
fully taught  with  books  alone ;  and  even  the  best  books  should  sup- 
plement, but  not  precede  or  take  the  place  of,  actual  observation. 

Specimens  to  illustrate  Guide  No.  XII.,  comprising  the  twenty 
principal  elements  and  minerals,  are  supplied  in  durable,  covered  boxes, 
properly  labelled,  as  follows  :  — 

I  large  specimen  of  each  kind,  20  in  all,  labelled  .     .     .  $  .50 

5  smaller  specimens  of  each  kind,   100     "  "          ...     1.25 

10       "  "  "  "        200     "  "          ...     2.25 

20       "  "  "          "       400    "          "          ...    4.00 


SCIENCE. 


Ten  additional  varieties  are  supplied  in  the  same  way :  — 

I  large  specimen  of  each  kind,  10  in  all,  labelled   .     .     .  $  .30 

5  smaller  specimens  of  each  kind,      50     "  ...       .75 

10        "  "  "  "        ioo     "  "  ...     1.50 

20          "  "  "  "          200      "  "  ...       2.50 

Orders  for  these  specimens  should  be  addressed  to  Prof.  W.  O. 
CROSBY,  Boston  Society  of  Natural  History ,  Boston^  Mass. 

First  Lessons  in  Minerals.     (No.  XIII.) 

By  ELLEN  H.  RICHARDS,  Instructor  in  Mineralogy,  Massachusetts  Insti- 
tute of  Technology.  4^  by  6  inches.  Paper.  50  pages.  Introduction 
price,  10  cents.  A  valuable  introduction  to  Guide  No.  XII. 

The  outline  of  the  lessons  was  first  worked  out  with  three  successive 
classes  of  children,  from  six  to  eight  years  old,  just  out  of  the  Kinder- 
garten. The  lessons  were  then  given  to  classes  in  two  public  schools 
in  the  city  of  Boston.  During  the  two  years  which  have  since  elapsed, 
they  have  been  given  to  about  one  thousand  children  of  the  fourth 
classes  of  several  of  the  Boston  Grammar  Schools.  They  have  also 
been  adopted  by  teachers  in  other  places.  Such  changes  have  been 
made  as  experience  has  shown  to  be  desirable,  and  the  Guide  is  now 
presented  in  a  form  which  can  be  recommended  to  teachers  in  general. 

The  specimens  to  illustrate  Guide  No.  XIII.  consist  of  large, 
carefully  selected  cabinet  specimens,  with  printed  labels.  It  is  desirable, 
however,  to  have  a  specimen  of  each  type  for  every  pupil,  or  at  least 
for  every  two  or  three  pupils.  To  meet  this  need,  duplicate  collections 
of  somewhat  smaller  specimens,  numbered  but  not  labelled,  have  been 
prepared. 

50  specs.       80  specs.       125  specs.      150  specs. 

Cabinet  size,  $2.00  $4.00  $8.00  $10.00 

Student  size,    2-5  colls.         i.oo  ea.          2.00  ea.          4.00  ea.          5.00  ea. 
"         "      6-10    "  .90  ea.  1. 80  ea.          3.60  ea.  4.50  ea. 

The  student  collections  are  not  sold  singly. 

Other  collections,  adapted  to  more  extended  courses,  are  supplied  as 
follows :  — 

Minerals.  50  specs.  ioo  specs.  150  specs. 

Cabinet  size,  $6.00  $15.00  $3°  °° 

Student  size,  2.00  5.00  10.00 


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UNIVERSITY  OF  CALIFORNIA  LIBRARY 


